The present invention relates to a jig and in particular to a surgical jig for use in determining an axis of a body part. More particularly, the surgical jig may be used for determining an axis extending into the end of a bone, and in particular a long bone such as a femur or a humerus.
For many surgical procedures and operations it can be necessary to determine an axis of a body part having a complex, often non-uniform, geometry. This can be achieved by the surgeon visually assessing the body part to determine where, and at what angle, to start drilling, cutting or carrying out any other invasive surgical procedure. This approach is limited in its accuracy and its successful utilisation can be highly dependent on the skill and experience of the surgeon. The results of a badly carried out procedure can be exacerbated in surgical procedures in which a correct determination of the axis position is important in order to ensure the correct mechanical functioning of the body part in the overall mechanical functioning of the body.
In the area of orthopaedic implants, if an axis is incorrectly determined intra-operatively then the implant can be incorrectly positioned which can give rise to various difficulties such as incorrect patient limb motion, pain, incorrect mechanical functioning of the implant, excessive wear of the implant and further damage to the body part. Therefore it would be desirable to be able to provide a device for assisting in the accurate determination of an axis of a body part.
An example of a situation in which it is important to accurately determine an axis of a body part intra-operatively is during the implantation of a prosthetic hip joint. It will be appreciated however that similar considerations are equally applicable to the implantation of other orthopaedic implants, for instance the humeral component of a shoulder joint. Hip prostheses are generally formed from two components: an acetabular, cup which lines the acetabulum, and a femoral component which replaces the femoral head. To implant the femoral component, the femoral head of the bone is shaped to receive the prosthesis. A stem part of the femoral component is inserted into the bore.
It is important that that the bearing surface of the femoral component is accurately positioned to mimic the original bone bearing surface. This requires that the hollowed cavity to receive the stem part of the femoral component is accurately aligned for both angular orientation and translational position within the femur. As an initial step it is known to use a surgical jig coupled to the head of the femur in order align a guide pin inserted into the head of the femur. Typically the guide pin is inserted along the femoral neck axis or at a predetermined offset to the femoral neck axis.
Known surgical jigs for implanting a guide pin into the head of a femur comprise a tripod arrangement in which three or more legs extend around the femoral head and clamp onto the sides of the femoral neck. Coupled to the legs is an alignment guide comprising a tube for positioning and inserting the guide pin into the femoral head. The desired position of the pin can be determined from pre-operative analysis of x-rays of the joint. The guide pin is left protruding from the femoral head to guide further surgical steps. A cannulated drill may be used to drill a bore into the end of the bone along the femoral neck axis guided by the guide pin. The drilled bore may then be used to guide a resection of the femoral head, ensuring that the resection is at a predetermined orientation relative to the femoral neck axis.
However, known surgical jigs based upon a tripod arrangement supported upon the surface of the femoral head can be unstable. Partly, this instability may be due to the location of clamping to the femoral neck being remote from the point of entry of the guide pin into the femoral head. Furthermore, as the position of the tripod is adjusted, the axis of the alignment guide coupled to the tripod shifts about a centre of rotation within the centre of the femoral neck between the clamps. This is undesirable as it results in the axis of the alignment guide upon the surface of the femoral head simultaneously changing angular orientation and translating across the surface of the bone.
It is an object of embodiments of the present invention to obviate or mitigate one or more of the problems associated with the prior art, whether identified herein or elsewhere. In particular, it is an object of embodiments of the present invention to provide an improved surgical jig which is arranged to couple to the end of a long bone to provide a more stable platform for determining an axis extending into the end of the bone. Furthermore, it is an object of embodiments of the present invention to provide a surgical jig incorporating an alignment guide for inserting a guide pin into the end of the bone in which the angle and position of the insertion point of the guide pin may be separately controlled.